As is well known (B. Born and E. Wolf, Principles of Optics (Pergamon Press, Oxford, 1980)), the resolving power of a microscope or optical lithographic system is determined by the numerical aperture, N.A., and the wavelength .lambda. of light used, that is ##EQU1## where the numerical aperture N.A.=n sin .theta., the angle .theta. is the collection angle subtended by the lens, .mu.=wavelength, and n is the index of refraction of the medium in which the lens is immersed.
In the prior art, efforts directed toward increasing resolution have focused on using shorter wavelength radiation. However, this presents obvious difficulties, for example, in the use of ultraviolet and X-ray optics, the ionization produced by such short wavelength radiation, and the like.
As an alternative to using shorter wavelength radiation, resolution can be enhanced by increasing the numerical aperture (Id.). This is the principle of the oil immersion microscope, in which the sample to be studied is essentially immersed in oil in order to increase n. However, since 1.0&lt;n&lt;1.5, the numerical aperture can at most be around 1.5, and thus only a modest increase in resolution is achieved.
It has long been known, however, that the index of refraction, n, can be made large by working in the neighborhood of an optical resonance. However, dispersion-absorption relations state that the absorption of the light will be overwhelming when the resonant index of refraction is large. The real part of the polarization (which governs the index of refraction n) is large where the detuning frequency .DELTA.=.gamma. (decay rate) but the absorption is also large at this point.
However, it is possible to obtain a large index of refraction n without large absorption. This is accomplished via the "atomic diode" concept herein disclosed in which atoms emit light but do not absorb. As such, the atomic diode concept is an extension of the "nonabsorbing" or "dark states" of matter first observed in the 1970's. That work demonstrated that it is possible to produce such nonabsorbing states of matter via quantum interference and atomic coherence. These ideas form the basis of current research involving lasers which operate without population inversion (S. Harris, Phys. Rev. Lett. 62, 1022 (1989); M. Scully, S. Zhu, and A Gavrieliedes, Phys. Rev. Lett. 62, 2813 (1989)); and have been observed in "Nonabsorption Resonance" experiments (G. Alzetta, A. Gozzini, L. Moi and G. Orriols: Nuovo Cimento 36B, 5 (1976); G. Alzetta in Coherence in Spectroscopy and Modern Physics, edited by F. T. Arecchi, R. Bonifacio and M. O. Scully (New York, N.Y., 1978); G. Alzetta, L. Moi and G. Orriols, Nuovo Cimento 52B, 209 (1979)) and "Electromagnetically Induced Transparency" experiments (K. Boller, A. Imamoglu, and S. Harris, "Observation of Electromagnetically Induced Transparency," Phys. Rev. Lett., to be published).
Laser radiation has been injected into a sodium vapor cell and the scattered resonant radiation observed. It was found that, under certain conditions, the atoms did not absorb the laser radiation. This state of gas was called "nonabsorption resonance."
More recently, optically thick media has been rendered transparent by applying a laser to excited atomic states so as to induce a destructive interference.
The present disclosure indicates that it is possible to generate a medium in which there is a large index of refraction, n, and a vanishingly small absorption.